Carrier wave television system



INVENTOR.

Alfred WhiIalC er;

H/S A TTORN E Y.

Patented June 12, 19.34

1,962,417 CARRIER WAVE TELEVISION SYSTEM Alfred Whitaker,

Gerrards Cross, England, as-

signor to The Gramophone Company Limited, Hayes, Middlesex, England, a company of Great Britain Application August 27, 1931, Serial No.

In Great Britain September 12, 1930 lclaim.

'I'he present invention relates to a carrier wave television system.

In known methods of transmitting images to a distant point a single carrier wave of comparatively high frequency, say 105 cycles, is modulated in accordance with the light and shade values of elements of the images, that is to say, is modulated with a bandl of frequencies which may vary from the picture-scanning frequency up to 1 some higher frequency determined by the dimensions of the scanning apparatus, and the same carrier wave is modulated with a single frequency for synchronizing purposes. Freque'ntly the synchronizing impulses are transmitted in the intervals between picture impulses.

According to the present invention, for the electrical transmission of images there are generated and transmitted two carrier waves of different frequencies, the beats produced by interference between the two carrier waves being utilized to control the speed of scanning apparatus both at the transmitting'and at the receiving ends. One of the carrier Waves may be modulated by electrical impulses corresponding to the light and shade values of elements of the images, and the other carrier wave is usually transmitted in an unmodulated form.

'I'he invention will now be described, by way of example, with reference to the accompanying drawing, in which Fig. 1 illustrates, diagrammatically, a television transmitter arranged in accordance with the present invention, and Fig. 2 illustrates, also diagrammatically, a television receiver arranged in accordance with the present invention. I

'I'he transmitting and receiving ends of the system are soarranged that two similar types of scanning devices may be used at each point. These scanning devices are conventionally shown by the numerals 11 and 12 on Figs. 1 and 2 respectively, and may be of any known or suitable form. For example, it may be assumed that the devices comprise a single mirror oscillated about two perpendicular axes under the inuence of lelectrical impulses of a predetermined frequency for example, or they may comprise other types of well known scanning apparatus 4such as disks, mirror wheels, moving tapes, cathode ray apparatus or the like.

To illustrate the system by. reference to one type of scanning apparatus, if the device is an oscillating mirror, a beam of light is reflected from the oscillating mirror on to the object of which the image is to be transmitted and is reflected therefrom upon a suitable light translating device formed as a part of the picture Scanning apparatus 11, such as, for example, a photoelectric device. Electric current impulses generated in the light-sensitive device are, after amplification in the ampnner 1, utilized to modulate a carrier@ wave. of frequency n cycles per second generated by the radio frequency generator 2. The picture signals are superimposed upon the carrier wave by way of the transformer 3, and this modulated carrier wave is transmitted in the usual manner 65 after amplification in the amplifier 4. w

A second radio frequency generator 5 generates a carrier wave of frequency (n-i-f) or (1L-j). The two carrier waves generated at 2 and 5 thus differ in frequency by an Iamount f which is the .'70 frequency suitable for maintaining the picture scanning apparatus at desired speed. The two generators 2 and 5 are connected, in parallel, to a radio frequency detector 6and it will be apparent that there must be produced, by interference between the two carrier waves, a beat of frequency f. The output from the detector is therefore passed through a ltering circuit 7 which is adapted to pass only oscillations of frequency f, and the oscillations emerging from the filter cir- 180 cuit 7 are utilized, if necessary after amplification, for regulating the scanning apparatus speed control device 13. This is accomplished by supl plying the beat frequency wave from the filter circuit 'I to the speed control apparatus 13. The 85 carrier wave, of frequency (n4-f) or (1i-f), generated in 5, is also transmitted, in an unmodulated form, after amplification in the amplifier 4.

At the receiving end, Fig. 2, the` two carrier waves, one modulated by the picture signals and the other unmodulated, are detected and amplifled by the radio frequency detector 8. The output from the detector 8 is passed in parallel through two filter circuits 9 and 10. The filter circuit 9, which is similar of Fig. 1, is adapted tc pass only the beat of frequency f produced by interference between the two carrier waves, and the output from the filter circuit 10 is utilized, in any known or suitable manner, to inuence the picture reproducing apparatus speed control 14 sp as to maintain synchronism' between the picture 'scanning apparatus 11 at the transmitter end of the system and the picture reproducingapparatus 12 at the receiver end of the system. 'Ihe lter circuit 10 105 is adapted to pass the whole of the wide band of frequencies comprising the picture signals which are necessary for the transmission ofvftictures and these signals may then be applied to any suitable form of light modulating device such as, for

to the lter circuit 7 95 example, a Kerr cell, or directly to a light source such as a glow tube and in either case the modulating light emitted is then directed by means of the picture reproducing apparatus ofy a form similar to that of the apparatus 11 at the transmtting end ofthe system to reproduce the picture and to maintain synchronism of operation in accordance with the beat frequency of the synchronizing wave.

From. the foregoing it will be apparent that the arrangement herein disclosed provides an improved system for television wherein two carrier waves of different frequencies are generated independently of the operation of the scanning apparatus at either the transmitting or the receiving station and that the beat frequency between these two waves is then utilized for controlling the operation of the scanning apparatus at both the transmitting and thel receiving stations.

It will be understood that the invention is in,

no wise limited to the type of scanning apparatus described herein, but that it is applicable to many types of scanning apparatus in which there are required synchronizing impulses at the transmitting andreceiving ends.

I claim as my invention:

Apparatus for the electrical transmission of images comprising means for generating two separate radio frequency carrier waves of different frequencies, a scanning device of a movable type at both the transmitter and receiver ends of the system; and means for producing beat frequency oscillations by interference between the two carrier waves to the control of the movements of both of lsaid devices, said generating means operating independently of operating action of said scanning devices.

ALFRED WHITAKER. 

